Porous thin films are widely used as optical filters, anti-reflection coatings, anti-fog surfaces, sensors, dielectrics and heat transfer surfaces. Typically, porous thin films contain air in the void regions, and such air-filled voids influence the properties of the films. For example, applications for porous films include dielectrics where the presence of air in the voids increases the composite dielectric constant. Porous thin films also provide for a volume, where air can be replaced by an alternative fluid. In the case of nanoporous thin films, where the typical size of a pore is smaller than a quarter wave of incident light, an anti-fog film can be produced. In other cases, liquid crystals, drugs, or dyes can also be interred into the porous film.
Porous thin films can be fabricated from vacuum based processes (e.g., chemical vapor deposition), created from sol-gel processes, electrostatic spray processes or assembled from nanoparticle solutions. Recently, complex porous thin films have been assembled from nanoparticle suspensions using a thin film technique called “layer-by-layer assembly” with wide flexibility over film morphology and composition as well as substrate shape and composition. The process creates films by taking advantage of self-limiting complementary interactions, such as electrostatic pairs or hydrogen bonding donors and acceptors. A major drawback of the technique however arises from the nature of the complementary interactions used to assemble the film which do not provide significant mechanical or environmental strength for many applications, such as optics.
A number of techniques have been investigated to improve the mechanical robustness of porous thin films. Chemical crosslinking, such as the thermally induced formation of amide bonds between carboxylic acid and amine functionalities, has been widely used but has demonstrated insufficient abrasion resistance for medium wear applications. Other post processing treatments such as thermal calcination or hydrothermal treatments have improved wear resistance but also to a limited degree. U.S. Pat. No. 5,925,228 describes a method for filling cracks and defects in dielectric, ceramic, and semiconductor coatings using electrophoretically active sol-gel preparations, but does not describe how to preferentially fill the pores over the surface.
To date, there has been little to no evidence that porous thin films from the layer by layer technique can survive the durability requirements needed for commercial applications. There is a need in the art for a method of significantly improving the mechanical properties of these films, while simultaneously maintaining the desired functional properties and advantages of porous thin films. Ideal methods and materials would exhibit one or more of the following advantages: utilize inexpensive and readily available materials; be applicable to a wide variety of porous thin films without preference for any particular film-forming method; lead to significantly enhanced film properties such as wear/abrasion resistance and toughness; and avoid causing significant degradation of desirable film properties such as transparency.